/*
 * Copyright 2002-2017 the original author or authors.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package org.springframework.jdbc.datasource;

import java.lang.reflect.InvocationHandler;
import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Method;
import java.lang.reflect.Proxy;
import java.sql.Connection;
import java.sql.SQLException;
import javax.sql.DataSource;

import org.apache.commons.logging.Log;
import org.apache.commons.logging.LogFactory;

import org.springframework.core.Constants;
import org.springframework.lang.Nullable;

/**
 * Proxy for a target DataSource, fetching actual JDBC Connections lazily,
 * i.e. not until first creation of a Statement. Connection initialization
 * properties like auto-commit mode, transaction isolation and read-only mode
 * will be kept and applied to the actual JDBC Connection as soon as an
 * actual Connection is fetched (if ever). Consequently, commit and rollback
 * calls will be ignored if no Statements have been created.
 *
 * <p>This DataSource proxy allows to avoid fetching JDBC Connections from
 * a pool unless actually necessary. JDBC transaction control can happen
 * without fetching a Connection from the pool or communicating with the
 * database; this will be done lazily on first creation of a JDBC Statement.
 *
 * <p><b>If you configure both a LazyConnectionDataSourceProxy and a
 * TransactionAwareDataSourceProxy, make sure that the latter is the outermost
 * DataSource.</b> In such a scenario, data access code will talk to the
 * transaction-aware DataSource, which will in turn work with the
 * LazyConnectionDataSourceProxy.
 *
 * <p>Lazy fetching of physical JDBC Connections is particularly beneficial
 * in a generic transaction demarcation environment. It allows you to demarcate
 * transactions on all methods that could potentially perform data access,
 * without paying a performance penalty if no actual data access happens.
 *
 * <p>This DataSource proxy gives you behavior analogous to JTA and a
 * transactional JNDI DataSource (as provided by the Java EE server), even
 * with a local transaction strategy like DataSourceTransactionManager or
 * HibernateTransactionManager. It does not add value with Spring's
 * JtaTransactionManager as transaction strategy.
 *
 * <p>Lazy fetching of JDBC Connections is also recommended for read-only
 * operations with Hibernate, in particular if the chances of resolving the
 * result in the second-level cache are high. This avoids the need to
 * communicate with the database at all for such read-only operations.
 * You will get the same effect with non-transactional reads, but lazy fetching
 * of JDBC Connections allows you to still perform reads in transactions.
 *
 * <p><b>NOTE:</b> This DataSource proxy needs to return wrapped Connections
 * (which implement the {@link ConnectionProxy} interface) in order to handle
 * lazy fetching of an actual JDBC Connection. Use {@link Connection#unwrap}
 * to retrieve the native JDBC Connection.
 *
 * @author Juergen Hoeller
 * @see DataSourceTransactionManager
 * @since 1.1.4
 */
public class LazyConnectionDataSourceProxy extends DelegatingDataSource {

    /**
     * Constants instance for TransactionDefinition
     */
    private static final Constants constants = new Constants(Connection.class);

    private static final Log logger = LogFactory.getLog(LazyConnectionDataSourceProxy.class);

    @Nullable
    private Boolean defaultAutoCommit;

    @Nullable
    private Integer defaultTransactionIsolation;


    /**
     * Create a new LazyConnectionDataSourceProxy.
     *
     * @see #setTargetDataSource
     */
    public LazyConnectionDataSourceProxy() {
    }

    /**
     * Create a new LazyConnectionDataSourceProxy.
     *
     * @param targetDataSource the target DataSource
     */
    public LazyConnectionDataSourceProxy(DataSource targetDataSource) {
        setTargetDataSource(targetDataSource);
        afterPropertiesSet();
    }


    /**
     * Set the default auto-commit mode to expose when no target Connection
     * has been fetched yet (-> actual JDBC Connection default not known yet).
     * <p>If not specified, the default gets determined by checking a target
     * Connection on startup. If that check fails, the default will be determined
     * lazily on first access of a Connection.
     *
     * @see java.sql.Connection#setAutoCommit
     */
    public void setDefaultAutoCommit(boolean defaultAutoCommit) {
        this.defaultAutoCommit = defaultAutoCommit;
    }

    /**
     * Set the default transaction isolation level to expose when no target Connection
     * has been fetched yet (-> actual JDBC Connection default not known yet).
     * <p>This property accepts the int constant value (e.g. 8) as defined in the
     * {@link java.sql.Connection} interface; it is mainly intended for programmatic
     * use. Consider using the "defaultTransactionIsolationName" property for setting
     * the value by name (e.g. "TRANSACTION_SERIALIZABLE").
     * <p>If not specified, the default gets determined by checking a target
     * Connection on startup. If that check fails, the default will be determined
     * lazily on first access of a Connection.
     *
     * @see #setDefaultTransactionIsolationName
     * @see java.sql.Connection#setTransactionIsolation
     */
    public void setDefaultTransactionIsolation(int defaultTransactionIsolation) {
        this.defaultTransactionIsolation = defaultTransactionIsolation;
    }

    /**
     * Set the default transaction isolation level by the name of the corresponding
     * constant in {@link java.sql.Connection}, e.g. "TRANSACTION_SERIALIZABLE".
     *
     * @param constantName name of the constant
     * @see #setDefaultTransactionIsolation
     * @see java.sql.Connection#TRANSACTION_READ_UNCOMMITTED
     * @see java.sql.Connection#TRANSACTION_READ_COMMITTED
     * @see java.sql.Connection#TRANSACTION_REPEATABLE_READ
     * @see java.sql.Connection#TRANSACTION_SERIALIZABLE
     */
    public void setDefaultTransactionIsolationName(String constantName) {
        setDefaultTransactionIsolation(constants.asNumber(constantName).intValue());
    }


    @Override
    public void afterPropertiesSet() {
        super.afterPropertiesSet();

        // Determine default auto-commit and transaction isolation
        // via a Connection from the target DataSource, if possible.
        if (this.defaultAutoCommit == null || this.defaultTransactionIsolation == null) {
            try {
                Connection con = obtainTargetDataSource().getConnection();
                try {
                    checkDefaultConnectionProperties(con);
                } finally {
                    con.close();
                }
            } catch (SQLException ex) {
                logger.warn("Could not retrieve default auto-commit and transaction isolation settings", ex);
            }
        }
    }

    /**
     * Check the default connection properties (auto-commit, transaction isolation),
     * keeping them to be able to expose them correctly without fetching an actual
     * JDBC Connection from the target DataSource.
     * <p>This will be invoked once on startup, but also for each retrieval of a
     * target Connection. If the check failed on startup (because the database was
     * down), we'll lazily retrieve those settings.
     *
     * @param con the Connection to use for checking
     * @throws SQLException if thrown by Connection methods
     */
    protected synchronized void checkDefaultConnectionProperties(Connection con) throws SQLException {
        if (this.defaultAutoCommit == null) {
            this.defaultAutoCommit = con.getAutoCommit();
        }
        if (this.defaultTransactionIsolation == null) {
            this.defaultTransactionIsolation = con.getTransactionIsolation();
        }
    }

    /**
     * Expose the default auto-commit value.
     */
    @Nullable
    protected Boolean defaultAutoCommit() {
        return this.defaultAutoCommit;
    }

    /**
     * Expose the default transaction isolation value.
     */
    @Nullable
    protected Integer defaultTransactionIsolation() {
        return this.defaultTransactionIsolation;
    }


    /**
     * Return a Connection handle that lazily fetches an actual JDBC Connection
     * when asked for a Statement (or PreparedStatement or CallableStatement).
     * <p>The returned Connection handle implements the ConnectionProxy interface,
     * allowing to retrieve the underlying target Connection.
     *
     * @return a lazy Connection handle
     * @see ConnectionProxy#getTargetConnection()
     */
    @Override
    public Connection getConnection() throws SQLException {
        return (Connection) Proxy.newProxyInstance(
                ConnectionProxy.class.getClassLoader(),
                new Class<?>[]{ConnectionProxy.class},
                new LazyConnectionInvocationHandler());
    }

    /**
     * Return a Connection handle that lazily fetches an actual JDBC Connection
     * when asked for a Statement (or PreparedStatement or CallableStatement).
     * <p>The returned Connection handle implements the ConnectionProxy interface,
     * allowing to retrieve the underlying target Connection.
     *
     * @param username the per-Connection username
     * @param password the per-Connection password
     * @return a lazy Connection handle
     * @see ConnectionProxy#getTargetConnection()
     */
    @Override
    public Connection getConnection(String username, String password) throws SQLException {
        return (Connection) Proxy.newProxyInstance(
                ConnectionProxy.class.getClassLoader(),
                new Class<?>[]{ConnectionProxy.class},
                new LazyConnectionInvocationHandler(username, password));
    }


    /**
     * Invocation handler that defers fetching an actual JDBC Connection
     * until first creation of a Statement.
     */
    private class LazyConnectionInvocationHandler implements InvocationHandler {

        @Nullable
        private String username;

        @Nullable
        private String password;

        private Boolean readOnly = Boolean.FALSE;

        @Nullable
        private Boolean autoCommit;

        @Nullable
        private Integer transactionIsolation;

        private boolean closed = false;

        @Nullable
        private Connection target;

        public LazyConnectionInvocationHandler() {
            this.autoCommit = defaultAutoCommit();
            this.transactionIsolation = defaultTransactionIsolation();
        }

        public LazyConnectionInvocationHandler(String username, String password) {
            this();
            this.username = username;
            this.password = password;
        }

        @Override
        @Nullable
        public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
            // Invocation on ConnectionProxy interface coming in...

            if (method.getName().equals("equals")) {
                // We must avoid fetching a target Connection for "equals".
                // Only consider equal when proxies are identical.
                return (proxy == args[0]);
            } else if (method.getName().equals("hashCode")) {
                // We must avoid fetching a target Connection for "hashCode",
                // and we must return the same hash code even when the target
                // Connection has been fetched: use hashCode of Connection proxy.
                return System.identityHashCode(proxy);
            } else if (method.getName().equals("unwrap")) {
                if (((Class<?>) args[0]).isInstance(proxy)) {
                    return proxy;
                }
            } else if (method.getName().equals("isWrapperFor")) {
                if (((Class<?>) args[0]).isInstance(proxy)) {
                    return true;
                }
            } else if (method.getName().equals("getTargetConnection")) {
                // Handle getTargetConnection method: return underlying connection.
                return getTargetConnection(method);
            }

            if (!hasTargetConnection()) {
                // No physical target Connection kept yet ->
                // resolve transaction demarcation methods without fetching
                // a physical JDBC Connection until absolutely necessary.

                if (method.getName().equals("toString")) {
                    return "Lazy Connection proxy for target DataSource [" + getTargetDataSource() + "]";
                } else if (method.getName().equals("isReadOnly")) {
                    return this.readOnly;
                } else if (method.getName().equals("setReadOnly")) {
                    this.readOnly = (Boolean) args[0];
                    return null;
                } else if (method.getName().equals("getTransactionIsolation")) {
                    if (this.transactionIsolation != null) {
                        return this.transactionIsolation;
                    }
                    // Else fetch actual Connection and check there,
                    // because we didn't have a default specified.
                } else if (method.getName().equals("setTransactionIsolation")) {
                    this.transactionIsolation = (Integer) args[0];
                    return null;
                } else if (method.getName().equals("getAutoCommit")) {
                    if (this.autoCommit != null) {
                        return this.autoCommit;
                    }
                    // Else fetch actual Connection and check there,
                    // because we didn't have a default specified.
                } else if (method.getName().equals("setAutoCommit")) {
                    this.autoCommit = (Boolean) args[0];
                    return null;
                } else if (method.getName().equals("commit")) {
                    // Ignore: no statements created yet.
                    return null;
                } else if (method.getName().equals("rollback")) {
                    // Ignore: no statements created yet.
                    return null;
                } else if (method.getName().equals("getWarnings")) {
                    return null;
                } else if (method.getName().equals("clearWarnings")) {
                    return null;
                } else if (method.getName().equals("close")) {
                    // Ignore: no target connection yet.
                    this.closed = true;
                    return null;
                } else if (method.getName().equals("isClosed")) {
                    return this.closed;
                } else if (this.closed) {
                    // Connection proxy closed, without ever having fetched a
                    // physical JDBC Connection: throw corresponding SQLException.
                    throw new SQLException("Illegal operation: connection is closed");
                }
            }

            // Target Connection already fetched,
            // or target Connection necessary for current operation ->
            // invoke method on target connection.
            try {
                return method.invoke(getTargetConnection(method), args);
            } catch (InvocationTargetException ex) {
                throw ex.getTargetException();
            }
        }

        /**
         * Return whether the proxy currently holds a target Connection.
         */
        private boolean hasTargetConnection() {
            return (this.target != null);
        }

        /**
         * Return the target Connection, fetching it and initializing it if necessary.
         */
        private Connection getTargetConnection(Method operation) throws SQLException {
            if (this.target == null) {
                // No target Connection held -> fetch one.
                if (logger.isDebugEnabled()) {
                    logger.debug("Connecting to database for operation '" + operation.getName() + "'");
                }

                // Fetch physical Connection from DataSource.
                this.target = (this.username != null) ?
                        obtainTargetDataSource().getConnection(this.username, this.password) :
                        obtainTargetDataSource().getConnection();

                // If we still lack default connection properties, check them now.
                checkDefaultConnectionProperties(this.target);

                // Apply kept transaction settings, if any.
                if (this.readOnly) {
                    try {
                        this.target.setReadOnly(true);
                    } catch (Exception ex) {
                        // "read-only not supported" -> ignore, it's just a hint anyway
                        logger.debug("Could not set JDBC Connection read-only", ex);
                    }
                }
                if (this.transactionIsolation != null &&
                        !this.transactionIsolation.equals(defaultTransactionIsolation())) {
                    this.target.setTransactionIsolation(this.transactionIsolation);
                }
                if (this.autoCommit != null && this.autoCommit != this.target.getAutoCommit()) {
                    this.target.setAutoCommit(this.autoCommit);
                }
            } else {
                // Target Connection already held -> return it.
                if (logger.isDebugEnabled()) {
                    logger.debug("Using existing database connection for operation '" + operation.getName() + "'");
                }
            }

            return this.target;
        }
    }

}
